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Sweetpotato is widely produced by smallholder farmers in Mozambique with average yield of 7.3 t/ha. One of the challenges in sweetpotato production system is inadequate seed during the rainy season resulting in low productivity. The few sweetpotato seed multipliers (Decentralized vine multipliers (DVMs)) resort to several vine harvesting frequency to meet farmers seed requirements at the peak of demand. Repeated vine harvesting contributes to sweetpotato root yield reduction. The objective of the study was to evaluate the effect of vine harvesting frequency on sweetpotato yield components. Three experiments were set up in three farmers’ fields in Vilankulos district inhambane province in Southern Mozambique in the 2021/2022 and 2022/2023 cropping seasons with a sweetpotato variety Esther. The experiments were set up in randomized complete block design with three replications in each farmer’s field. The treatments were no vine harvesting (control), harvesting only once at 8 weeks after planting (WAP), harvesting twice at 8 and 12 WAP, and harvesting three times at 8, 12, and 16 WAP. Total storage root yield, number of roots per plant, root diameter, and root length were measured in each treatment. Results show that storage root yield was significantly higher in control (22.1 t/ha) and harvesting once (17.8 t/ha) compared to harvesting twice (11.8 t/ha) and three times (9 t/ha) (P < 0.05). Additional harvesting from one to two resulted in 33.7% yield decline. Vine harvesting twice resulted in 46.6% storage root yield decline compared to the control treatment. The same observation was noted for the number of roots per plant, where five roots per plant were observed both in no harvesting and harvesting once treatments. A 40% decline in the number of roots per plant was observed when vine harvesting frequency was increased from one to two. No treatment effect was observed on the root diameter. Increasing vine harvesting frequency from one to two or one to three resulted in a 14.7% income loss from sales of both vines and roots. Vine multipliers are recommended to harvest vines for use as planting material only once if they want to benefit from the roots for consumption or commercialization.

Introduction

Smallholder farmers in southern Mozambique meet their food security requirements through crops from their fields. Sweetpotato (Ipomoea batatas (L) Lam) belonging to the Convolvulaceae family [1] is one such important food security crop. Globally, sweetpotato is the 7th most important after rice, wheat, potato, maize, cassava, and barley, of which about 96.3% is produced in developing countries, with Asia accounting for 82.3% and Africa contributing about 14% of the production [2], [3]. Sweetpotato is grown in over 8.62 million ha and in over 100 countries in the world with an annual production of about 105 metric tons [4] [2]. In Mozambique, sweetpotato is ranked the third most-produced crop after cassava and maize [5]. Sweetpotato is widely grown because of its wide ecological adaptation [2] and drought tolerance, and it produces relatively good yields in poor and marginal soils [6]. Sweetpotato rapidly covers the soil surface, hence helping to reduce soil erosion [7]. The roots are edible, such as boiled or roasted, and can be processed into a puree that is used in the bakery industry (baking bread, scones, and doughnuts) or can be processed into flour. In addition, sweetpotato leaves are eaten as vegetables while the whole plant can be used as forage. The apical plant portion (vines) is used as planting material. Sweetpotato can be produced continuously throughout the year and therefore serve as a reliable source of carbohydrates, protein, iron, vitamins B and C. The orange-fleshed sweetpotato varieties are rich in β-carotene, which can be converted by the human body into vitamin A.

One of the challenges facing sweetpotato farmers in Mozambique is seed availability at the onset of the planting/rainy season (hereafter referred to as vines). Most farmers lose planting material due to drying up because of long dry periods and being eaten by domestic animals during the off-season period. Few sweetpotato decentralized vine multipliers (DVMs) who conserve planting material in protected humid lowlands multiply vines and sell to the rest of the farmers at the onset of the rainy season. Recent trends in Mozambique show an increase in commercialization of sweetpotato vines for use as planting material. At the same time, the same DVMs are equally interested in harvesting sweetpotato storage roots for household consumption or for sale. Therefore, vines and fresh roots are increasingly becoming important sources of income in rural farming communities in Mozambique. However, the quantity of vines produced by DVMs does not meet the demand by farmers, especially at the peak of sweetpotato planting season. This forces the few multipliers with smaller vine multiplication areas to harvest vines more frequently from the same plants to meet the high vine demand from the farmers.

Frequent removal of sweetpotato vines as planting material or leaves and shoots as vegetables, or forage might be expected to interfere with photosynthesis thereby reducing the amount of photosynthates from the source (leaves) to the sinks (roots) [8]. The extent of storage root development and yield in sweetpotato is largely influenced by the processes of photosynthesis that take place in the vegetative portion of the plant, mainly leaves. Frequent harvesting of the vegetative plant parts decreases storage root yield due to reduced photosynthates production and subsequent low quantity translocated to the roots. Previous studies by Aniekwe [6] and Indawan et al. [7] reported that harvesting sweetpotato vines at 4 weeks after planting (4 WAP) reduced the growth and yield parameters. Storage root yield is still the most important factor to be considered when deciding on how and when to harvest sweetpotato vines for use as planting material or for animal feeding. There is a need, therefore, to carefully determine an appropriate vine harvesting frequency to overcome the problem of low seed availability at the period of highest demand without losing on the final sweetpotato storage root yield. Limited knowledge is available on the implication of frequent sweetpotato vine harvesting for use as planting material by farmers on sweetpotato yield in Mozambique. The objective of this study was to determine the effect of vine harvesting frequency (for use as planting material) on the yield components of sweetpotato and to identify optimum level of vine harvesting frequency that maintains reasonable sweetpotato root yield and gross income from sweetpotato roots and vines commercialization in southern Mozambique.

Materials and Methods

Site

The experiment was conducted at three farmers’ fields, namely Inacio Thumbo, Azarias Muabsa, and Samuel Chibebe. The location, altitude, temperature, and average rainfall of the experimental sites are shown in Table I.

Site GPS location Altitude (m) Average temperature (°C) Average annual rainfall (mm)
Inacio Thumbo 22° 07′ 21″ S 35° 11′ 08″ E 27 26 1500
Azarias Muabsa 22° 13′ 06″ S 35° 10′ 38″ E 34 26 1500
Sameul Chibebe 22° 06′ 35″ S 35° 10′ 29″ E 26 26 1500
Table I. Location and Altitude of the Experimental Sites

The soil chemical and physical properties at the experimental site are shown in Table II.

pHH2O (1:2.5) K+ P N Cation exchange capacity (CEC)
Producer name/site [−] [meq/100 g] [mg/kg] [%] [meq/100 g]
Inácio Tumbo 8.36 1.67 2.91 0.02 17.7
Azarias Muabsa 7.89 0.45 2.91 0.266 7.82
Sameul Chibebe 7.77 0.43 4.00 0.013 10.87
Table II. Soil Chemical Characteristics at the Three Experimental Sites

Generally, the soil at Inacio Tumbo’s farm has higher potassium content of 1.67 meq/100 g compared with soils at Azarias Muabsa and Samuel Chibebe, with Potassium levels of 0.45 meq/100 g and 0.43 meq/100 g, respectively (Table II). Potassium is a very important macronutrient necessary for bulking in sweetpotato production system. The soil at Inacio Tumbo’s farm is relatively higher in CEC (17.7 meq/100 g) compared with the soils at Azarias and Samuel’s farms with CEC of 7.82 meq/100 g and 10.87 meq/100 g, respectively.

Experimental Design

This study was based on a participatory action research approach. Community-based on-farm trials were established in farmers’ fields as technology learning, transfer, and adoption centers [8], [9]. Three field experiments were set up in Vilankulos district in southern Mozambique on three farmer’s fields. The experiments were set up in a randomized complete block design with three replications in each farmer’s field. The experimental treatments were no vine harvesting (control), vine harvesting only once at 8 weeks after planting, vine harvesting two times at 8 and 10 weeks after planting and harvesting vines three times at 8, 10, and 12 weeks after planting. Planting was done during the 2021/2022 and 2022/23 cropping seasons under rain-fed conditions. Sweetpotato plants were planted on ridges measuring 90 cm between ridges and 30 cm between plants within each ridge, equivalent to a plant population of 37,037 plants per hectare (ha). Weeding was done uniformly across the plots, and no fertilizer was applied to mimic the normal production practice in the experimental area. An area of 7.5 m2 net plot was harvested. A small group of 40 farmers from each of the experimental sites was invited to participate in each of the vine harvesting treatments as participatory research. At harvest, a field day was conducted with the same 40 farmers participating at each site to collect data and observe the results from each treatment plot. Therefore, a total of 120 farmers participated in the field days to collect data and observe the results from each treatment plot.

Measured Parameters

At harvest, five plants were randomly selected from the net plot (7.5 m2) and removed from the soil and the number of roots from each plant was physically counted, and an average number of roots per plant was determined. From the same randomly selected plants, 10 roots were randomly selected, and root length and diameter were measured using a pair of calipers. Total storage root yield was determined by weighing all the harvested storage roots from the net plot using an electronic balance and converting them on a per-hectare basis. Commercial root yield was determined by selecting and weighing undamaged roots, roots free from insect (weevil) damage, and big-sized roots of at least 200 g in weight as required by the local market. At each of the harvesting frequencies, harvested vines from the net plot were weighed (g), and the data was used to determine vine yield per hectare. Total gross income from roots and vines was determined as follows: Total vine yield in kg ×10 meticais (price per kg) plus total root yield in kg ×25 meticais (price of roots per kg).

Statistical Analysis

Data was subjected to normality test using Shapiro-Wilk at 5% of significance. Analysis of variance (ANOVA) was conducted using R programming. Whenever the ANOVA showed a significant difference, the Least Significance Difference (LSD) at the 5% level of confidence was used to separate the treatment means.

Results

Quantity of Vine Harvested for Use as Planting Material

The results of the study indicated that zero harvesting did not produce any vines for use as planting material (Fig. 1). However, harvesting vines three times resulted in a total of 24 t/ha, which was 41.2% more than harvesting vines only twice, which yielded 17 t/ha. Additionally, harvesting vines three times for use as planting material (yielding 24 t/ha) resulted in more than double the number of vines harvested by harvesting only once (that yielded 10 t/ha). Furthermore, harvesting vines twice (that yielded 17 t/ha) resulted in 70% more vine yield compared to vine harvesting only once (that yielded 10 t/ha).

Fig. 1. Average total vine yield (t/ha) as influenced by vine harvesting frequencies in Southern Mozambique.

Sweetpotato Total Storage Root Yield

Total storage root yield was significantly affected by vine harvesting frequency in all farmers’ fields (P < 0.05) (Fig. 2). More importantly, vine harvesting two or three times significantly reduced sweetpotato total storage root yield compared to either no harvesting or harvesting once in all the experimental sites (P < 0.05) (Fig. 2). Vine harvesting only once resulted in an average storage root yield of 17.8 t/ha, while the no-harvesting treatment resulted in an average storage root yield of 22.1 t/ha that was not significantly different from each other (P > 0.05) (Fig. 2). Similarly, there was no significant root yield difference between harvesting vines two and three times, with two vine harvestings resulting in an average root yield of 11.8 t/ha compared to three vine harvesting frequencies that resulted in 9.0 t/ha. The results indicate a yield decline of 46.6% by harvesting two times compared to the control. Vine harvesting twice resulted in a 33.7% yield decline compared to plots whose vines were harvested only once. A further harvesting to three times resulted in 59.3% storage root yield decline compared to the control. Although there was no significant total root yield difference, harvesting vines only once for use as planting material resulted in a slight yield decline of 19.4% compared to the control treatment.

Fig. 2. Sweetpotato total root yield (t/ha) as influenced by vine harvesting frequency.

Sweetpotao Commercial Storage Root Yield

Commercial storage root yield refers to the yield of roots that are saleable (free from pest damage with a size that is acceptable by the market (>200 g size roots)). Results of the current study showed that commercial root yield was highest in the control treatment (19.8 t/ha) (Fig. 3). This yield was, however, not significantly different from the yield achieved in treatment where harvesting was done once (P > 0.05). Sweetpotato vine harvesting two or three times significantly depressed commercial root yield (P < 0.05) (Fig. 3). The commercial root yield under these two treatments was significantly lower than the control (19.8 t/ha) or the vine harvesting once treatment (15.9 t/ha) (P < 0.05). Harvesting twice and three times resulted in a 55% and 66.2% commercial root yield decline, respectively, from the control treatment.

Fig. 3. Sweetpotato commercial root yield (t/ha) as influenced by vine harvesting frequency.

Number of Storage Roots Per Plant

The lowest number of roots per plant (three) was observed where vine harvesting was done two or three times, and there was no significant difference in the number of roots per plant for these two harvesting frequencies (P > 0.05) (Fig. 4). However, significantly higher number of roots per plant were observed in plots where harvesting was not done or where vine harvesting was done only once than where vine harvesting was done twice or three times (P < 0.05) (Fig. 4). No difference in the number of roots per plant was observed in no vine harvesting plots and in plots where harvesting was done once. In addition, vine harvesting twice and three times resulted in no significant difference in the number of roots per plant (P > 0.05). Harvesting vines three times resulted in a 40% decline in the number of roots per plant compared to harvesting vines once for use as planting material (Fig. 4).

Fig. 4. Sweetpotato number of roots per plant as influenced by vine harvesting frequency.

Storage Root Length

The study results indicate that, on average, the storage root length was affected by vine harvesting frequency. Sweetpotato plots without any vine harvesting had an average of 16.9 cm root length, which was significantly higher than roots from plots whose vines were cut three times (P < 0.05) (Fig. 5). Harvesting vines three times resulted in a 25.4% reduction in root length compared to the control treatment (Fig. 5). However, no significant difference in storage root length was observed for sweetpotato roots harvested from plots that suffered vine harvesting (P > 0.05). Storage root lengths of 13.7 cm, 13.5 cm, and 12.6 cm were observed from roots harvested from plots whose vines were cut once, twice, and thrice, respectively.

Fig. 5. Sweetpotato storage root length as influenced by vine harvesting frequency.

Gross Income from Sweetpotato Vine and Storage Root Sales

Results from gross income analysis from vine and storage root sales indicate that by not harvesting vines, the farmers got a gross income of 495,000 meticais from sweetpotato root sales (Table III). The gross income from root sales was the second highest a farmer could achieve in treatment plots where vines were not harvested. However, by harvesting vines once, there was a slight root yield decline to 17.8 t/ha and a total vine yield of 10 t/ha, resulting in a slight increase (0.5%) in total gross income to 497,500 meticais. A further increase in the number of vines harvesting frequency to two and three times resulted in 20.7% and 17.7% declines in gross income, respectively, compared to the control treatment. Results indicated that increasing vine harvesting frequency beyond one led to gross income loss from combined sales of both vines and roots.

Number of harvestings Vine yield (t/ha) Vine selling price/kg Total income from vine sales (Meticais) Commercial root yield (t/ha) Roots selling price/kg Total income from root sale (Meticais) Total gross income vine and root sales (Meticais)
No vine harvesting 0 0 0 19.8 25 495,000 495,000
Vine harvesting only once 10 10 100,000 15.9 25 397,500 497,500
Vine harvesting twice 17 10 170,000 8.9 25 222,500 392,500
Vine harvesting three times 24 10 240,000 6.7 25 167,500 407,500
Table III. Income from Vine and Roots Sales from Following Different Vine Harvesting Frequencies

Discussion

The purpose of sweetpotato vine harvesting mainly by decentralized vine multipliers (DVMs) is for use as planting material. DVMs sell vines to farmers in their communities. In recent years, sweetpotato vine commercialization has increasingly become a lucrative business in Mozambique. However, frequent vine harvesting has a negative impact on storage root yield because of it interferes with interception of solar radiation that in turn interfers with the process of photosynthesis. Photosynthesis is a process that is responsible for the synthesis of sugars and oxygen using sunlight, carbon dioxide, and water by green plants. In the current study, increasing harvesting frequency from one to two resulted in a storage root yield decline of 33.3% and a 40% decline in the number of roots per plant. A further increase in the number of vine harvesting frequencies from two to three further reduced total storage root yield by 23.7%. Similar results were reported by Snapp [10], who observed 15.8% cassava yield decline because of cassava leaf removal in DR Congo. Decreased storage root yield observed in this study can be explained by the decline in photosynthetic translocation to the storage root possibly due to a diversion of the photosynthates to promote vine regrowth at the expense of its translocation to the sink (storage roots). According to Hauser et al. [11], increasing the frequency of vine harvests increases the partitioning of assimilates to vines. Similar studies by Gomes et al. [12] in Bangladesh and Uddin et al. [13] in Tanzania indicated that defoliation had a significant depressing influence on storage root production in sweetpotato because of interference with photosynthetic processes. The same pattern of results was reported by Kiozya et al. [14], who noted a 63% storage root yield decrease because of shoot removal to within 20 cm from the ground during several harvesting periods. These findings can also be explained by the fact that storage root bulking in sweetpotato is a function of both sink capacity and source potential, which are associated with photosynthesis. The vegetative parts, including leaves and stems, are the source and the storage roots as the sinks where photosynthetic products are stored [15]. Removal of the vegetative portion of sweetpotato interferes with the process of photosynthesis that equally affects the number of roots per plant, storage root length, and ultimate storage root yield. Similarly, Aniekwe [6] reported that harvesting sweetpotato vine as early as 4 weeks after planting reduced growth and yield because of strong interruption in the physiological processes of growth and storage root bulking. The results of the current study show that harvesting vines only once does not influence both the number of roots per plant and storage root yield. This is probably because harvesting vines once 60 days after planting is still within the period in which storage root formation and development is taking place, and the plant can still recover and proceed with photosynthetic activities. Increasing vine harvesting frequency to two or three times removes ground cover, which is critical for moisture conservation. The exposure of soil in treatments where vine harvesting was done more than once probably accelerated water loss through evaporation, resulting in poor root formation, hence low yield, low number of roots per plant, and low storage root length.

Plant vegetative parts, especially leaves, are the main centers of photosynthetic activities and, therefore, act as sources of photosynthesis. On the other hand, the roots of sweetpotato plants are primary storage organs and act as sinks to photosynthetic products, where deposition of different phytochemicals, phytonutrients, and carbohydrates take place [16]. Maintenance of sweetpotato leaves and stems (vines) is critical in the capture of solar radiation necessary for the process of photosynthesis and other biochemical processes within plant cells. This, in turn, is critical for the maintenance of the source-sink relationship. Source–sink balance is an ideal state reached in the late growth stage of sweetpotato growth, when the photosynthetic efficiency of the source (vines, leaves and stems) and proportion of synthesized photosynthate distributed to the storage roots are highest. Interference of a balanced source-sink relationship through repeated vine harvesting for use as planting material prevents sufficient growth of source (leaves), which would reduce photosynthetic efficiency and hence reduce the proportion of photosynthates distributed to storage roots, resulting in low yield components. Any interference in the vegetative plant parts negatively affects the bulking of storage roots and the number of roots per plant in sweetpotato. Prolonged removal of the vegetative part during the mid and late-growth stages limits canopy development and the total number of leaves to capture solar radiation, thereby reducing photosynthetic activity, which ultimately affects storage root development, storage root bulking, and ultimate root yield [17]. The degree of yield loss depends on the vine harvesting frequency as demonstrated in this study where significant decline in sweetpotato storage root and commercial root yield and number of roots per plant was observed in treatment plots where harvesting was done two or three times when compared to plots where no harvesting was done.

Increasing the number of vine harvestings affects both subsequent vine yield and root yield, resulting in a decline in gross income.

Recommendations

Based on the results of the current study it is recommended that farmers interested in both storage root yield and vines for sale or for use as planting material should only harvest vines once at 60 days after planting. It is further recommended that farmers with interest in both vines for sale and roots should consider having two separate fields, one solely for harvesting vines with little or no expectation of income or reasonable yield from storage root yield and a separate field whose vines are not cut as planting materials for the sole purpose of root production.

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